This invention relates to a flexible coupling for transmitting torque from a drive shaft to a driven shaft and which attenuates the transmission of noise and/or vibration by or through a drive member and a driven member.
Flexible couplings are used where there is likely to be misalignment between coaxially aligned members of a power train, for example, between the engine and the propeller shaft of a ship. One form of known flexible coupling comprises a drive member and driven member which include inter-engaging means in the form of interdigitating radial vanes which are circumferentially spaced to which flexible load transmitting elements are attached and interposed between corresponding vanes. The flexible load transmitting elements, which typically comprise pads made from a number of layers of resilient deformable material interposed by rigid spacer plates, transmit torque from the vanes of the drive member to the driven member. The pads, being resiliently deformable, minimise any noise and/or vibration transmission by or through the coupling. Whilst the known coupling arrangements are found to work effectively the inventors have appreciated that they have certain limitations.
For example, in the case of ships it is often the case that the power required in the reverse (astern) direction of rotation is less than that required when operating in the forward (ahead) direction of rotation. For such applications a flexible coupling which was designed for equal power transmission in both directions of rotation would be over-engineered when the coupling operates in the astern sense and results in a coupling which is heavier than it need be. To overcome this problem it has been proposed to design couplings on an asymmetric basis, that is the coupling has different properties depending on the direction of rotation. For example, in one arrangement a larger number of pads is provided for operation in the principal direction (ahead) of rotation and fewer in the reverse direction (astem). To ensure that the coupling is as small as possible the resultant gaps between the interdigitating vanes where pads have been removed are reduced by altering the circumferential spacing of the vanes.
In an asymmetric coupling arrangement the way in which the pads are attached to the vanes depends on the direction of rotation. For example various techniques are known such as bonding or fitting the pads and spacer plates into slotted openings on the vanes. Typically the slots are dovetailed and the sets of pads slid into place on the vanes. The dovetail joint arrangement is expensive and time consuming to machine to close tolerances and requires considerable hand labour to obtain a proper fit. The inventors have appreciated that a particular disadvantage of this arrangement is that different vanes are required on the drive and driven members and these are particularly costly to manufacture. To improve the uniformity of compressional loading over the pad surface the pads are wedge shaped (radially tapered) with a taper angle of between 8xc2x0 and 11xc2x0, and are of greatest thickness at the circumference and least towards the axis of rotation. Such an arrangement ensures that the force per unit thickness of pad is substantially constant over the pad""s surface area. A disadvantage of this arrangement is that when a new coupling is required which can transmit a different amount of torque, a new set of pads has to be designed. In particular, the angle of taper of the pads is altered in dependence on the diameter of the drive or driven members to maintain a constant circular pitch which ensures a constant pad pressure. Since the pads have to undergo rigorous testing which can take many weeks (typically the pads are tested for 10 million cycles at a rate of rotation of a few hundred revolutions per minute), this significantly increases the cost of designing new couplings.
The present invention arose in an endeavour to provide a flexible coupling which at least in part overcomes the limitations of the known arrangements.
According to the present invention there is provided a flexible coupling for transmitting torque from a drive shaft to a driven shaft, the coupling comprising: a rotatable drive member securable to the drive shaft; a rotatable driven member securable to the driven shaft; a plurality of radial vanes on the drive member which interdigitate with a plurality of radial vanes on the driven member and resiliently deformable elements interposed between the interdigitating vanes to transmit torque there between; wherein the resiliently deformable elements are detachably secured to only either the vanes on the drive member or the vanes on the driven member and are removeable in a radial direction without having to remove the drive or driven vanes from the coupling.
Advantageously in its rest state none of the resiliently deformable elements are in a state of compression thereby enabling individual elements to be readily replaced whilst the coupling remains in situ. In contrast in the known couplings the resilienty deformable elements are secured to both the drive and driven vanes and are in a state of pre-compression to eliminate the element being in a state of tension when the coupling operates in a reverse direction of rotation. Since the elements are pre-compressed this makes it difficult, if not impossible, to replace or service the elements without having to remove one or more vanes and in many cases without having to dismantle the entire coupling which requires de-installation of the coupling.
In a much preferred arrangement the resiliently deformable elements for one direction of rotation, most preferably the principal direction of rotation of the coupling, are restrained from moving in a radial and axial direction but are free to move in a circumferential direction in relation to the corresponding vane to which it is not secured.
In a particularly preferred embodiment of the invention each vane has a substantially flat surface and an opposite surface having a channel formed therein, the vanes being arranged on the drive and driven members such that the channel in a vane on the drive member is used to detachably secure one face of the resiliently deformable element and the opposite face of the resiliently deformable element engages in the channel on the adjacent vane on the driven member to restrain its movement in the axial and radial directions.
Advantageously the radial vanes are detachably secured to the drive and driven members and are removeable in a radial direction without having to dismantle the coupling thereby further facilitating servicing and maintenance of the coupling.
Preferably the vanes on the drive and driven members are interchangeable and therefore the same vane can be used on both members of the coupling which significantly reduces the cost of the coupling.
Most conveniently the vanes are radially tapered and the resiliently deformable elements have substantially parallel faces. A particular advantage of using elements which have substantially parallel faces is that new couplings can be readily designed which utilise the same elements and thereby eliminate the need for expensive testing. Furthermore, having parallel faces enables the elements to be easily removed and replaced.
Advantageously the surface of the resiliently deformable elements in a circumferential plane of the respective rotatable member has a rounded perimeter. Due to the rounded shape of the resiliently deformable elements they are better able to more uniformly withstand any shear component of force in the plane of the elements as a result of coaxial misalignment of the drive and driven members. In contrast the known couplings use square or rectangular deformable elements and are consequently better able to withstand shear forces which act in a direction between the corners of the element than those which act between the edges.
Preferably the shape of the resiliently deformable elements has a continuously curved shape and is advantageously substantially circular or substantially elliptical in shape. It will be appreciated that elements which are substantially circular have uniform characteristics for all directions of shear.
Alternatively the shape of the resiliently deformable elements is polygonal and the junction between adjacent sides is curved, such as a square with rounded corners.
Preferably the resilienity deformable elements are cylindrical. Although there will be a slight overloading of the element on the outermost edge such an arrangement enables the same elements to be used even when the size of coupling changes which eliminates the need for expensive testing of the elements.
The resiliently deformable elements advantageously comprise a first set of elements which are compressed in a first direction of rotation and a second set of elements which are compressed in a second direction of rotation and wherein the vanes are equally spaced circumferentially on the drive and the driven members and there are unequal numbers of first and second resiliently deformable elements. This is particularly advantageous since a replacement set of elements can be fitted prior to removing the worn ones. Furthermore, the coupling can be readily adapted by adding or removing elements. This feature is considered inventive in its own right.
The resiliently deformable elements preferably comprise a plurality of resiliently deformable pads having rigid spacer plates interposed there between.
According to a second aspect of the invention a flexible coupling for transmitting torque from a drive shaft to a driven shaft, the coupling comprising: a rotatable drive member securable to the drive shaft; a rotatable driven member securable to the driven shaft; a plurality of radial vanes on the drive member which interdigitate with a plurality of radial vanes on the driven member and first and second resiliently deformable elements interposed between the interdigitating vanes to transmit torque there between wherein the first deformable elements are compressed in a first direction of rotation and the second deformable elements are compressed in a second direction of rotation; wherein the vanes are equally spaced circumferentially on the drive and the driven members and there are unequal numbers of first and second resiliently deformable elements.